A large number of liquid crystal display elements utilizing the optical (refractive index) anisotropy (Δn) (hereinafter, may be simply referred to as “Δn”) or the dielectric anisotropy (Δ∈) (hereinafter, may be simply referred to as “Δ∈”), which are characteristics of a liquid crystal compound, have been hitherto produced. Liquid crystal display elements are widely utilized in timepieces, calculators, various measuring instruments, automotive panels, word processors, electronic schedulers, mobile telephones, printers, computers, television sets and the like. Thus, the demand is also increasing every year. Liquid crystal compounds exhibit an intrinsic liquid crystal phase which is positioned intermediately between the solid phase and the liquid phase, and the phase morphology is broadly classified into a nematic phase, a smectic phase and a cholesteric phase. Among these, the nematic phase is currently most widely used for the use in display elements. Among the systems applied to liquid crystal display elements, representative examples of the display system include a TN (twisted nematic) type, an STN (super-twisted nematic) type, a DS (dynamic scattering) type, a GH (guest host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertically aligned) type, a CSH (color super homeotropic) type, or an FLC (ferroelectric liquid crystal) type. As the drive system, a multiplex drive system has become more common than the conventional static drive system, and a simple matrix system, and more recently, an active matrix (AM) system that is driven by a TFT (thin film transistor) or a MIM, have become the mainstream.
Conventionally, as the liquid crystal displays of the AM system, TN type liquid crystal display apparatuses in which a liquid crystal material having positive dielectric anisotropy (Δ∈) is oriented horizontally to the substrate surface, such that the liquid crystal material is twisted 90° between facing substrates, have been the mainstream. However, this TN type has a problem that the viewing angle is narrow, and thus extensive investigations have been conducted to promote a widening of the viewing angle.
As a system that is converted to this TN type, there have been developed the VA type, among others, the MVA type and EVA type as described in Non-Patent Literature 1 or Non-Patent Literature 2, and it has been successful in improving the viewing angle characteristics to a large extent. These are systems in which a liquid crystal material having negative dielectric anisotropy is vertically oriented between two substrates, and the tilt direction of the liquid crystal molecules at the time of voltage application is regulated by the protrusions or slits provided on the surface of the substrates. Recently, as described in Patent Literature 1, liquid crystal display apparatuses in which a liquid crystal material containing a monomer or oligomer that is polymerized by light or heat is encapsulated between substrates, and the polymerizable component is polymerized while the voltage to be applied to the liquid crystal layer is adjusted, thereby having the orientation direction of the liquid crystals determined, have also been suggested. Those apparatuses are called liquid crystal displays of a PS (polymer stabilized) type or a PSA (polymer sustained alignment) type. The technologies of PS(A) improve the trade-off between the light transmittance and the response speed in liquid crystal display apparatuses of an MVA type or the like, and therefore, development of a PSA-VA type (or a PS-VA type) is in progress.
As such, investigations have been conducted on the widening of the viewing angle, and these days, the application of the widening of the viewing angle to large-sized liquid crystal display apparatuses, which are represented by laptop computers, monitors, and television sets, has been made practical. However, those liquid crystal display elements have a drawback that the response speed to an electric field is slow, and there is still a significant demand in connection with the response speed. Especially, an improvement in the speed is demanded in television sets and the like that are used in a moving image display. In order to achieve a high-precision and high-resolution display with liquid crystal display elements, there is a need to develop and use a liquid crystal having a fast response speed.
Furthermore, in the case of driving a liquid crystal display element in a certain frame period, in order to obtain a display with satisfactory contrast, it is desirable that a liquid crystal display element have a high voltage holding ratio (voltage holding ratio during a frame period). Particularly, when energy such as light or heat is applied to a liquid crystal material as described in Patent Literature 1, there is a concern about a decrease in the voltage holding ratio, which results from deterioration of the compound. In regard to such problems, if the energy such as light or heat required to polymerize the polymerizable compound is reduced, deterioration of the compound is suppressed. Therefore, it is desirable that the polymerization reactivity of the polymerizable compound be high.
Further, if the polymerization reactivity of the polymerizable compound is low, the intended effect of acquiring a fast response speed of the liquid crystal display element is reduced. Also, as those unreacted polymerizable compound molecules undergo polymerization at a later time, there is a risk that the voltage-transmittance characteristics may change. Therefore, it is desirable that the polymerizable compound have high polymerization reactivity.
Patent Literature 1 discloses a VA liquid crystal display apparatus with reduced burn-in, and a composition used therein, and Patent Literatures 2 and 3 disclose polymerizable compounds having a trifunctional(meth)acryloyl group introduced as a polymerizable group, but these patent Literatures do not mention on the reactivity. Patent Literature 4 discloses alignment films which provide VA liquid crystal display elements having high voltage holding ratios; however, even if these are used, a satisfactory response speed could not be obtained. Patent Literatures 5 and 6 disclose VA liquid crystal compositions, but there is no description that a polymerizable compound having a (meth)acryloyl group introduced as a polymerizable group is used. Patent Literature 7 discloses a polymerizable compound, but the patent Literature neither describes nor suggests the use of the compound in a liquid crystal composition having negative dielectric anisotropy (Δ∈). Also, there is a problem that a compound having a trifunctional(meth)acryloyl group as a polymerizable group has poor solubility.
In PSA-VA type liquid crystal displays, the polymerizable compound is polymerized while a voltage is applied. That polymer takes the role as protrusions in the MVA type, and regulates the tilt direction of the molecules at the time of voltage application. Therefore, by implementing compartmentalization of the molecular orientation region, the viewing angle can be widened similarly to the MVA type and the like. Also, in the MVA type, the protrusions cause a decrease in the transparency to the incident light, and thus, improvements in the luminance and contrast have been desired. However, in the PSA type, since protrusions are not needed, those characteristics are enhanced, and also, simplification of the production process is made possible. In the PSA type, immobilization of the polymerizable compound is important, and for that purpose, high reactivity is required. However, those polymerizable compounds that are known to date may precipitate out in the liquid crystal composition due to their low solubility, or the polymerizable compounds have problems that the voltage holding ratio decreases, or that the intended effect of making the response speed of the display element fast is small, due to their low reactivity. Also, even if the reactivity of the polymerizable group is high, a VT shift (a change in the behavior of transmittance against voltage) occurs, so that the reliability of the liquid crystal display elements is decreased. Patent Literatures 8 to 10 disclose the technology of PS(A).